The aim of this proposal is to test in primates a new method for producing tolerance to solid organ grafts: implantation of purified donor CD34+ marrow cells into the recipient thymus. In pilot studies, the investigators have achieved: 1) hematopoietic microchimerism in 6/6 juvenile baboons with or without current triple drug immunosuppression; 2) specific prolongation of donor versus third party skin graft survival; and 3) absence of graft versus host disease at 16 months. These experiments test the feasibility of extending this approach to cardiac transplantation in a juvenile baboon model as a pre-clinical trial. The following questions are addressed: 1) Does intrathymic implantation of CD34+ marrow cells prolong survival of cardiac allografts? 2) What is the relationship between hematopoietic microchimerism and tolerance to a cardiac allograft? 3) Is intrathymic implantation of stem cells more effective than peripheral infusion? 4) What are the effects of current drug therapy on engraftment and tolerance? 5) What is the optimal timing of stem cell administration relative to cardiac transplantation? 6) What dose of cells is needed? 7) Are DR+ fractions necessary for engraftment? 8) Are measures of cellular immunity altered? Experiments are performed in juvenile baboons, implanting CD34+ cells in the thymus through a small thoracotomy and using a cervical heterotopic cardiac graft which facilitates biopsies. Immunosuppression consists of the tripe drug immunosuppressive regimen in current clinical practice, without myeloablation or host T cell depletion, or no immunosuppression. A dose of only 1x 10/6 cells has resulted in hematopoietic microchimerism in all recipients. Persistence of microchimerism. Histologic quantitation of infiltrating leukocyte subsets and coronary microvascular adhesion molecule expression will be compared to controls. The level of chimerism achieved, its persistence, the phenotype of stem cell progeny, effect on donor-directed CTLp, and the dependence on the thymus will be assessed. Implantation of CD34+ marrow cells in the thymus during a cardiac transplant procedure may provide a simple, safe method of producing tolerance to the graft and microchimerism. This methodology has obvious immediate clinical applications in solid organ transplantation. Depending on the level of chimerism achieved, the potential for introducing allogenic stem cells into an infant without immunosuppression might also have implications for the treatment of genetic deficiency diseases.